Photodiode detectors are widely used to detect and measure low level light signals. A common use of these photodiode detectors is as laser power meters.
When measuring the light from a source to be measured. photodiode detectors also measure the ambient background light. This results in readings which are higher than the light level emitted by the source to be measured. Moreover, since the background light level is not known, it is impossible to determine which portion of the measured light was emitted by the source to measured and which is measured background light. This problem can be alleviated by taking the measurements in a darkened room, but requiring a darkened room has the obvious disadvantage of severely limiting where the measurement can be made.
Another way of alleviating this problem is background subtraction. In background subtraction, a first measurement is made of only the background light, and the detector is zeroed at the measured light level. Then a second measurement is made of the light from the source to be measured and the background light, and since the detector is zeroed at the background level, the detector's output represents the light level of the source to be measured.
Background subtraction, however, requires that two measurements be taken in order to determine the light level of the source to be measured. Moreover, if the background level changes between the first measurement and the second measurement, the detector will provide inaccurate readings since the detector will be zeroed at the background level measured during the first measurement rather than at the actual background level present during the second measurement. The background level can change between the first and second measurement either because of changes in the background or because the detector is moved to a new location with a different background.